Methods and systems for forming images with radiation

ABSTRACT

Disclosed herein is a method comprising: obtaining a signal at a pixel in an array of pixels of a radiation detector, wherein the signal is generated from radiation incident on the radiation detector; obtaining a corrected signal by correcting the signal with a combination of a set of reference signals generated from the radiation at a set of reference pixels in the array, wherein a set of weights are respectively applied to the set of reference signals in the combination; and forming an image based on the corrected signal; wherein the set of weights is a function of a location of the pixel with respect to the array.

BACKGROUND

Radiation detectors may be devices used to measure the flux, spatialdistribution, spectrum or other properties of radiations.

Radiation detectors may be used for many applications. One importantapplication is imaging. Radiation imaging is a radiography technique andcan be used to reveal the internal structure of a non-uniformly composedand opaque object such as the human body.

Early radiation detectors for imaging include photographic plates andphotographic films. A photographic plate may be a glass plate with acoating of light-sensitive emulsion. Although photographic plates werereplaced by photographic films, they may still be used in specialsituations due to the superior quality they offer and their extremestability. A photographic film may be a plastic film (e.g., a strip orsheet) with a coating of light-sensitive emulsion.

In the 1980s, photostimulable phosphor plates (PSP plates) becameavailable. A PSP plate may contain a phosphor material with colorcenters in its lattice. When the PSP plate is exposed to radiation,electrons excited by radiation are trapped in the color centers untilthey are stimulated by a laser beam scanning over the plate surface. Asthe plate is scanned by laser, trapped excited electrons give off light,which is collected by a photomultiplier tube. The collected light isconverted into a digital image. In contrast to photographic plates andphotographic films, PSP plates can be reused.

Another kind of radiation detectors are radiation image intensifiers.Components of a radiation image intensifier are usually sealed in avacuum. In contrast to photographic plates, photographic films, and PSPplates, radiation image intensifiers may produce real-time images, i.e.,do not require post-exposure processing to produce images. Radiationfirst hits an input phosphor (e.g., cesium iodide) and is converted tovisible light. The visible light then hits a photocathode (e.g., a thinmetal layer containing cesium and antimony compounds) and causesemission of electrons. The number of emitted electrons is proportionalto the intensity of the incident radiation. The emitted electrons areprojected, through electron optics, onto an output phosphor and causethe output phosphor to produce a visible-light image.

Scintillators operate somewhat similarly to radiation image intensifiersin that scintillators (e.g., sodium iodide) absorb radiation and emitvisible light, which can then be detected by a suitable image sensor forvisible light. In scintillators, the visible light spreads and scattersin all directions and thus reduces spatial resolution. Reducing thescintillator thickness helps to improve the spatial resolution but alsoreduces absorption of radiation. A scintillator thus has to strike acompromise between absorption efficiency and resolution.

Semiconductor radiation detectors largely overcome this problem bydirect conversion of radiation into electric signals. A semiconductorradiation detector may include a semiconductor layer that absorbsradiation in wavelengths of interest. When a particle of radiation isabsorbed in the semiconductor layer, multiple charge carriers (e.g.,electrons and holes) are generated and swept under an electric fieldtowards electric contacts on the semiconductor layer. Cumbersome heatmanagement required in currently available semiconductor radiationdetectors (e.g., Medipix) can make a detector with a large area and alarge number of pixels difficult or impossible to produce.

SUMMARY

Disclosed herein is a method comprising: obtaining a signal at a pixelin an array of pixels of a radiation detector, wherein the signal isgenerated from radiation incident on the radiation detector; obtaining acorrected signal by correcting the signal with a combination of a set ofreference signals generated from the radiation at a set of referencepixels in the array, wherein a set of weights are respectively appliedto the set of reference signals in the combination; and forming an imagebased on the corrected signal; wherein the set of weights is a functionof a location of the pixel with respect to the array.

According to an embodiment, each pixel in the array of pixelsencompasses a portion of a radiation absorption layer of the radiationdetector.

According to an embodiment, the set of weights is a function of athickness of the radiation absorption layer.

According to an embodiment, the radiation absorption layer comprisessilicon.

According to an embodiment, the signal is generated from charge carriersproduced in the radiation absorption layer by the radiation.

According to an embodiment, the set of weights is a function of adirection of propagation of the radiation at the pixel.

According to an embodiment, the set of weights is a function of relativepositions of the set of reference pixels with respect to the pixel.

According to an embodiment, the pixel is a member of the set ofreference pixels.

According to an embodiment, the radiation is X-ray or gamma ray.

According to an embodiment, the signal and the set of reference signalsare generated during the same time period.

According to an embodiment, the combination is a sum of the set ofreference signals with the set of weights applied thereto.

According to an embodiment, the signal represents an intensity of theradiation at the pixel.

According to an embodiment, the radiation detector comprises a radiationabsorption layer and an electronics layer; wherein the radiationabsorption layer comprises an electrode; wherein the electronics layercomprises an electronics system; wherein the electronics systemcomprises: a first voltage comparator configured to compare a voltage ofthe electrode to a first threshold, a second voltage comparatorconfigured to compare the voltage to a second threshold, a counterconfigured to register a number of radiation photons reaching theradiation absorption layer, and a controller; wherein the controller isconfigured to start a time delay from a time at which the first voltagecomparator determines that an absolute value of the voltage equals orexceeds an absolute value of the first threshold; wherein the controlleris configured to activate the second voltage comparator during the timedelay; wherein the controller is configured to cause the numberregistered by the counter to increase by one, if the second voltagecomparator determines that an absolute value of the voltage equals orexceeds an absolute value of the second threshold.

According to an embodiment, the electronics system further comprises anintegrator electrically connected to the electrode, wherein theintegrator is configured to collect charge carriers from the electrode.

According to an embodiment, the controller is configured to activate thesecond voltage comparator at a beginning or expiration of the timedelay.

According to an embodiment, the electronics system further comprises avoltmeter, wherein the controller is configured to cause the voltmeterto measure the voltage upon expiration of the time delay.

According to an embodiment, the controller is configured to determine aradiation photon energy based on a value of the voltage measured uponexpiration of the time delay.

According to an embodiment, the controller is configured to connect theelectrode to an electrical ground.

According to an embodiment, a rate of change of the voltage issubstantially zero at expiration of the time delay.

According to an embodiment, a rate of change of the voltage issubstantially non-zero at expiration of the time delay.

Disclosed here is a computer program product comprising a non-transitorycomputer readable medium having instructions recorded thereon, theinstructions when executed by a computer implementing a method of anyone of above mentioned.

Disclosed herein is a system comprising: a radiation detector configuredto generate a signal at a pixel in an array of pixels of the radiationdetector from radiation incident on the radiation detector; a processorconfigured to obtain a corrected signal by correcting the signal with acombination of a set of reference signals generated from the radiationat a set of reference pixels in the array, wherein a set of weights arerespectively applied to the set of reference signals in the combination;wherein the processor is configured to form an image based on thecorrected signal; wherein the set of weights is a function of a locationof the pixel with respect to the array.

According to an embodiment, each pixel in the array of pixelsencompasses a portion of a radiation absorption layer of the radiationdetector.

According to an embodiment, the set of weights is a function of athickness of the radiation absorption layer.

According to an embodiment, the radiation absorption layer comprisessilicon.

According to an embodiment, the signal is generated from charge carriersproduced in the radiation absorption layer by the radiation.

According to an embodiment, the set of weights is a function of adirection of propagation of the radiation at the pixel.

According to an embodiment, the set of weights is a function of relativepositions of the set of reference pixels with respect to the pixel.

According to an embodiment, the pixel is a member of the set ofreference pixels.

According to an embodiment, the radiation is X-ray or gamma ray.

According to an embodiment, the signal and the set of reference signalsare generated during the same time period.

According to an embodiment, the combination is a sum of the set ofreference signals with the set of weights applied thereto.

According to an embodiment, the signal represents an intensity of theradiation at the pixel.

BRIEF DESCRIPTION OF FIGURES

FIG. 1A schematically shows a cross-sectional view of a radiationdetector, according to an embodiment.

FIG. 1B schematically shows a detailed cross-sectional view of thedetector, according to an embodiment.

FIG. 1C schematically shows an alternative detailed cross-sectional viewof the detector, according to an embodiment.

FIG. 2 schematically shows that the detector may have an array ofpixels, according to an embodiment.

FIG. 3 schematically shows that the pixels in the array may receiveparticles of radiation at different angles of incidence.

FIG. 4A schematically shows that charge carriers generated in multiplepixels when a particle of radiation with an oblique angle of incidencetravels through the radiation absorption layer.

FIG. 4B schematically shows that charge carriers generated in a singlepixel when a particle of radiation with an angle of incidence of 0°travels through the radiation absorption layer.

FIGS. 5A-5C each schematically show corrections of a signal from a pixelusing signals from reference pixels, according to some embodiments.

FIG. 6 schematically shows a functional diagram of a system comprisingthe radiation detector and a processor, according to an embodiment.

FIG. 7 schematically shows a flowchart for a method, according to anembodiment.

FIG. 8 schematically shows a system comprising the method describedherein, suitable for medical imaging such as chest radiationradiography, abdominal radiation radiography, etc., according to anembodiment.

FIG. 9 schematically shows a system comprising the method describedherein suitable for dental radiation radiography, according to anembodiment.

FIG. 10 schematically shows a cargo scanning or non-intrusive inspection(NII) system comprising the method described herein, according to anembodiment.

FIG. 11 schematically shows another cargo scanning or non-intrusiveinspection (NII) system comprising the method described herein,according to an embodiment.

FIG. 12 schematically shows a full-body scanner system comprising themethod described herein, according to an embodiment.

FIG. 13 schematically shows a radiation computed tomography (RadiationCT) system comprising the method described herein, according to anembodiment.

FIG. 14A and FIG. 14B each show a component diagram of an electronicsystem of the radiation detector in FIG. 1A, FIG. 1B and FIG. 1C,according to an embodiment.

FIG. 15 schematically shows a temporal change of the electric currentflowing through an electrode (upper curve) of a diode or an electriccontact of a resistor of a radiation absorption layer exposed toradiation, the electric current caused by charge carriers generated by aparticle of radiation incident on the radiation absorption layer, and acorresponding temporal change of the voltage of the electrode (lowercurve), according to an embodiment.

DETAILED DESCRIPTION

FIG. 1A schematically shows a cross-sectional view of a radiationdetector 100, according to an embodiment. The radiation detector 100 mayinclude a radiation absorption layer 110 and an electronics layer 120(e.g., an ASIC) for processing or analyzing electrical signals incidentradiation generates in the radiation absorption layer 110. In anembodiment, the radiation detector 100 does not comprise a scintillator.The radiation absorption layer 110 may include a semiconductor materialsuch as, silicon, germanium, GaAs, CdTe, CdZnTe, or a combinationthereof. The semiconductor may have a high mass attenuation coefficientfor the radiation energy of interest. The surface 103 of the radiationabsorption layer 110 distal from the electronics layer 120 is configuredto receive radiation. The radiation may be X-ray or gamma ray.

As shown in a detailed cross-sectional view of the radiation detector100 in FIG. 1B, according to an embodiment, the radiation absorptionlayer 110 may include one or more diodes (e.g., p-i-n or p-n) formed bya first doped region 111, one or more discrete regions 114 of a seconddoped region 113. The second doped region 113 may be separated from thefirst doped region 111 by an optional the intrinsic region 112. Thediscrete regions 114 are separated from one another by the first dopedregion 111 or the intrinsic region 112. The first doped region 111 andthe second doped region 113 have opposite types of doping (e.g., region111 is p-type and region 113 is n-type, or region 111 is n-type andregion 113 is p-type). In the example in FIG. 1B, each of the discreteregions 114 of the second doped region 113 forms a diode with the firstdoped region 111 and the optional intrinsic region 112. Namely, in theexample in FIG. 1B, the radiation absorption layer 110 has a pluralityof diodes having the first doped region 111 as a shared electrode. Thefirst doped region 111 may also have discrete portions.

When a particle of radiation hits the radiation absorption layer 110including diodes, the particle of radiation may be absorbed and generateone or more charge carriers by a number of mechanisms. A particle ofradiation may generate 10 to 100000 charge carriers. The charge carriersmay drift to the electrodes of one of the diodes under an electricfield. The field may be an external electric field. The electric contact119B may include discrete portions each of which is in electricalcontact with the discrete regions 114. In an embodiment, the chargecarriers may drift in directions such that the charge carriers generatedby a single particle of radiation are not substantially shared by twodifferent discrete regions 114 (“not substantially shared” here meansless than 2%, less than 0.5%, less than 0.1%, or less than 0.01% ofthese charge carriers flow to a different one of the discrete regions114 than the rest of the charge carriers). Charge carriers generated bya particle of radiation incident around the footprint of one of thesediscrete regions 114 are not substantially shared with another of thesediscrete regions 114. A pixel 150 associated with a discrete region 114may be an area around the discrete region 114 in which substantially all(more than 98%, more than 99.5%, more than 99.9%, or more than 99.99%of) charge carriers generated by a particle of radiation incidenttherein at an angle of incidence of 0° flow to the discrete region 114.Namely, less than 2%, less than 1%, less than 0.1%, or less than 0.01%of these charge carriers flow beyond the pixel.

As shown in an alternative detailed cross-sectional view of theradiation detector 100 in FIG. 1C, according to an embodiment, theradiation absorption layer 110 may include a resistor of a semiconductormaterial such as, silicon, germanium, GaAs, CdTe, CdZnTe, or acombination thereof, but does not include a diode. The semiconductor mayhave a high mass attenuation coefficient for the radiation energy ofinterest.

When a particle of radiation hits the radiation absorption layer 110including a resistor but not diodes, it may be absorbed and generate oneor more charge carriers by a number of mechanisms. A particle ofradiation may generate 10 to 100000 charge carriers. The charge carriersmay drift to the electric contacts 119A and 119B under an electricfield. The field may be an external electric field. The electric contact119B includes discrete portions. In an embodiment, the charge carriersmay drift in directions such that the charge carriers generated by asingle particle of radiation are not substantially shared by twodifferent discrete portions of the electric contact 119B (“notsubstantially shared” here means less than 2%, less than 0.5%, less than0.1%, or less than 0.01% of these charge carriers flow to a differentone of the discrete portions than the rest of the charge carriers).Charge carriers generated by a particle of radiation incident around thefootprint of one of these discrete portions of the electric contact 119Bare not substantially shared with another of these discrete portions ofthe electric contact 119B. A pixel 150 associated with a discreteportion of the electric contact 119B may be an area around the discreteportion in which substantially all (more than 98%, more than 99.5%, morethan 99.9% or more than 99.99% of) charge carriers generated by aparticle of radiation incident at an angle of incidence of 0° thereinflow to the discrete portion of the electric contact 119B. Namely, lessthan 2%, less than 0.5%, less than 0.1%, or less than 0.01% of thesecharge carriers flow beyond the pixel associated with the one discreteportion of the electric contact 119B.

The electronics layer 120 may include an electronic system 121 suitablefor processing or interpreting signals generated by particles ofradiation incident on the radiation absorption layer 110. The electronicsystem 121 may include an analog circuitry such as a filter network,amplifiers, integrators, and comparators, or a digital circuitry such asa microprocessor, and memory. The electronic system 121 may includecomponents shared by the pixels or components dedicated to a singlepixel. For example, the electronic system 121 may include an amplifierdedicated to each pixel and a microprocessor shared among all thepixels. The electronic system 121 may be electrically connected to thepixels by vias 131. Space among the vias may be filled with a fillermaterial 130, which may increase the mechanical stability of theconnection of the electronics layer 120 to the radiation absorptionlayer 110. Other bonding techniques are possible to connect theelectronic system 121 to the pixels without using vias.

FIG. 2 schematically shows that the radiation detector 100 may have anarray of pixels 150. Each of the pixels 150 may encompasses a portion ofthe radiation absorption layer 110. The array may be a rectangulararray, a honeycomb array, a hexagonal array or any other suitable array.Each pixel 150 in the array may be configured to detect a particle ofradiation incident thereon, measure the energy of the particle ofradiation, or both. For example, each pixel 150 may be configured tocount numbers of particles of radiation incident thereon whose energyfalls in a plurality of bins, within a period of time. All the pixels150 may be configured to count the numbers of particles of radiationincident thereon within a plurality of bins of energy within the sameperiod of time. Each pixel 150 may have its own analog-to-digitalconverter (ADC) configured to digitize an analog signal representing theenergy of an incident particle of radiation into a digital signal. TheADC may have a resolution of 10 bits or higher. Each pixel 150 may beconfigured to measure its dark current, such as before or concurrentlywith each particle of radiation incident thereon. Each pixel 150 may beconfigured to deduct the contribution of the dark current from theenergy of the particle of radiation incident thereon. The pixels 150 maybe configured to operate in parallel. For example, when one pixel 150measures an incident particle of radiation, another pixel 150 may bewaiting for another particle of radiation to arrive. The pixels 150 maybe but do not have to be individually addressable.

As shown in FIG. 3 , pixels 150 in the array may receive particles ofradiation from a radiation source 109 at different angles of incidencedue to different positions of the pixels 150 with respect to theradiation source 109. The spatial resolution of the radiation detector100 at different positions thereon may depend on the angle of incidenceat those positions. The spatial resolution may be lower where the angleof incidence is oblique (e.g., > 45°) than the spatial resolution wherethe angle of incidence is 0°. For example, the spatial resolution at theedges of the radiation detector 100 may be lower than the spatialresolution at the center of the radiation detector 100 when the angle ofincidence at the center is 0° and the angles of incidence at the edgesare oblique. If the field of view of the radiation detector 100 is large(e.g., 0.5 π or larger) or the thickness of the radiation absorptionlayer 110 is comparable to or larger than the size of the pixels 150,the decrease of the spatial resolution from the center to the edges canbe significant. The field of view of the radiation detector 100 is asolid angle through which the radiation detector 100 is sensitive to theradiation. FIG. 4A schematically shows that charge carriers may begenerated in the pixels associated with multiple discrete portions ofthe electric contact 119B when a particle of radiation with an obliqueangle of incidence travels through the radiation absorption layer 110.FIG. 4B schematically shows that charge carriers may be generated in thepixel associated with a single discrete portion of the electric contact119B when a particle of radiation with an angle of incidence of 0°travels through the radiation absorption layer 110.

The signal generated at a pixel in the array from the incident radiation(e.g., due to the effects shown in FIG. 4A) may be corrected with acombination of a set of reference signals generated from the radiationat a set of reference pixels in the array. A set of weights may berespectively applied to the set of reference signals in the combination.The combination may be expressed using a formula C = ƒ({e}, {p}), whereC is the combination, {e} is the set of reference signals and {p} is theset of weights. The set of weights may be a function of a location ofthe pixel with respect to the array. In the example shown in FIG. 5A, tocorrect the signal from the pixel 150A, the set of reference pixels forthe pixel 150A may include neighboring pixels (e.g., R1, R2, R3, R4, R5,R6, R9 and R10 in FIG. 5A), or pixels at particular locations within thearray. The pixel 150A itself may be a member of the set of referencepixels. The set of reference signals may respectively be generated atthe set of reference pixels during the same time period as the signalgenerated at the pixel 150A. The set of weights associated with thepixel 150A is respectively applied to the set of reference signals tocorrect the signal of the pixel 150A. The set of weights may be afunction of relative locations of the reference pixels with respect tothe array. The combination of the set of reference signals may be a sumof the set of reference signals with the set of weights applied thereto.For example, the corrected signal of the pixel 150A may be E_(150A) =e_(150A) + [(e_(150A) × p_(150A)) + (e_(R9) × p_(R9)) + (e_(R10) ×p_(R10)) + (e_(R1) × p_(R1)) + (e_(R2) × p_(R2)) + (e_(R3) × p_(R3)) +(e_(R4) × p_(R4)) + (e_(R5) × p_(R5)) + (e_(R6) × p_(R6))], wheree_(150A) is the signal of the pixel 150A, the set of reference signalsinclude the signal e_(150A) of the pixel 150A, the signal e_(R9) of thepixel R9, the signal e_(R10) of the pixel R10, the signal e_(R1) of thepixel R1, the signal e_(R2) of the pixel R2, the signal e_(R3) of thepixel R3, the signal e_(R4) of the pixel R4, the signal e_(R5) of thepixel R5, and the signal e_(R6) of the pixel R6, and p_(150A) is theweight applied to e_(150A), p_(R9) is the weight applied to e_(R9),p_(R10) is the weight applied to e_(R10), p_(R1) is the weight appliedto e_(R1), p_(R2) is the weight applied to e_(R2), p_(R3) is the weightapplied to e_(R3), p_(R4) is the weight applied to e_(R4), p_(R5) is theweight applied to e_(R5), p_(R6) is the weight applied to e_(R6).

According to an embodiment, the set of weights is a function of adirection of propagation of the radiation at the pixel, which is insidethe direction of propagation of the radiation inside the radiationabsorption layer 110 at the pixel. The direction of propagation of theradiation may be related to the angle of incidence. In the example shownin FIG. 5B, the angle of incidence of the particle of radiation receivedby the pixel 150A is oblique (e.g., > 45°) and the direction ofpropagation is also not perpendicular to the surface of the radiationdetector 100. The particle of radiation may generate charges carries inother pixels (e.g., R2, R7, R8) before it reaches the pixel 150A. Inthis scenario, the set of reference pixels may include the pixels (e.g.,R2, R7, R8) along the traveling path of the particle of radiation. Theset of reference pixels may also include other neighboring pixels (e.g.,R1, R3, R4, R5, R6, R9, R10). The set of the reference pixels may alsoinclude the pixel 150A itself. For example, the corrected signal of thepixel 150A may be E_(150A) = e_(150A) + (e_(150A) × p_(150A)) + (e_(R9)× p_(R9)) + (e_(R10) × p_(R10)) + (e_(R1) × p_(R1)) + (e_(R2) ×p_(R2)) + (e_(R3) × p_(R3)) + (e_(R4) × p_(R4)) + (e_(R5) × p_(R5)) +(e_(R6) × p_(R6)) + (e_(R7) × p_(R7)) + (e_(R8) × p_(R8))], where p_(R7)is the weight applied to e_(R7), p_(R8) is the weight applied to e_(R8).

According to an embodiment, the set of weights is a function of relativepositions of the set of reference pixels with respect to the pixel. Inthe example shown in FIG. 5B, the particle of radiation generates morecharge carriers in reference pixels R7 and R8 than in reference pixel R5because by the time the particle reaches the pixel 150A, it has beenalmost entirely absorbed and thus any signal it generates in referencepixel R5 is expected to be much weaker than the signals it generates inreference pixels R7 and R8. The weights for reference pixel R7 and R8thus might be larger than the weight for reference pixel R5.

According to one embodiment, the set of weights is a function of athickness of the radiation absorption layer of the radiation detector100. In FIG. 5C, a radiation detector 101 with a thicker radiationabsorption layer than the radiation detector 100 is shown. At thedirection of propagation, a particle of radiation may pass through morepixels in the thicker radiation absorption layer. Therefore, the set ofreference pixels may include more than the immediately neighboringpixels. For example, the set of reference pixels may include referencepixels R1-R6, R9, R10, and R11-R26. The set of weights thus wouldinclude weights for reference pixels R11-R26.

FIG. 6 schematically shows a system 9000, according to an embodiment.The system has the radiation detector 100 and a processor 115 configuredto execute the corrections on the signals from one or more pixels in theradiation detector 100. The processor 115 may be configured to form animage 1100 based on the corrected signals.

FIG. 7 schematically shows a flowchart for a method, according to anembodiment. In procedure 151, a signal at a pixel in an array of pixelsof a radiation detector is obtained (e.g., signal e_(150A) of the pixel150A of the radiation detector 100). The signal is generated fromradiation incident on the radiation detector 100. In procedure 152, acorrected signal is obtained by correcting the signal with a combinationof a set of reference signals generated from the radiation at a set ofreference pixels in the array. A set of weights are respectively appliedto the set of reference signals in the combination. The set of weightsis a function of a location of the pixel (e.g., a position of the pixel150A) with respect to the array. In procedure 153, an image is formedbased on the corrected signal (e.g., the corrected signal E_(150A)). Themethod may be implemented by executing instructions using a computer,where the instructions are recorded on a computer program productcomprising a non-transitory computer readable medium.

The radiation detector 100 described above may be used in varioussystems such as those provided below.

FIG. 8 schematically shows a system comprising the radiation detector100 described herein. The system may be used for medical imaging such aschest radiation radiography, abdominal radiation radiography, etc. Thesystem comprises a radiation source 1201. Radiation emitted from theradiation source 1201 penetrates an object 1202 (e.g., a human body partsuch as chest, limb, abdomen), is attenuated by different degrees by theinternal structures of the object 1202 (e.g., bones, muscle, fat andorgans, etc.), and is projected to radiation detector 100. The radiationdetector 100 forms an image by detecting the intensity distribution ofthe radiation.

FIG. 9 schematically shows a system comprising the radiation detector100 described herein. The system may be used for medical imaging such asdental radiation radiography. The system comprises a radiation source1301. Radiation emitted from the radiation source 1301 penetrates anobject 1302 that is part of a mammal (e.g., human) mouth. The object1302 may include a maxilla bone, a palate bone, a tooth, the mandible,or the tongue. The radiation is attenuated by different degrees by thedifferent structures of the object 1302 and is projected to theradiation detector 100. The radiation detector 100 forms an image bydetecting the intensity distribution of the radiation. Teeth absorbradiation more than dental caries, infections, periodontal ligament. Thedosage of radiation received by a dental patient is typically small(around 0.150 mSv for a full mouth series).

FIG. 10 schematically shows a cargo scanning or non-intrusive inspection(NII) system comprising the radiation detector 100 described herein. Thesystem may be used for inspecting and identifying goods intransportation systems such as shipping containers, vehicles, ships,luggage, etc. The system comprises a radiation source 1401. Radiationemitted from the radiation source 1401 may backscatter from an object1402 (e.g., shipping containers, vehicles, ships, etc.) and be projectedto the radiation detector 100. Different internal structures of theobject 1402 may backscatter radiation differently. The radiationdetector 100 forms an image by detecting the intensity distribution ofthe backscattered radiation and/or energies of the backscatteredparticles of radiation.

FIG. 11 schematically shows another cargo scanning or non-intrusiveinspection (NII) system comprising the radiation detector 100 describedherein. The system may be used for luggage screening at publictransportation stations and airports. The system comprises a radiationsource 1501. Radiation emitted from the radiation source 1501 maypenetrate a piece of luggage 1502, be differently attenuated by thecontents of the luggage, and projected to the radiation detector 100.The radiation detector 100 forms an image by detecting the intensitydistribution of the transmitted radiation. The system may revealcontents of luggage and identify items forbidden on publictransportation, such as firearms, narcotics, edged weapons, flammables.

FIG. 12 schematically shows a full-body scanner system comprising theradiation detector 100 described herein. The full-body scanner systemmay detect objects on a person’s body for security screening purposes,without physically removing clothes or making physical contact. Thefull-body scanner system may be able to detect non-metal objects. Thefull-body scanner system comprises a radiation source 1601. Radiationemitted from the radiation source 1601 may backscatter from a human 1602being screened and objects thereon, and be projected to the radiationdetector 100. The objects and the human body may backscatter radiationdifferently. The radiation detector 100 forms an image by detecting theintensity distribution of the backscattered radiation. The radiationdetector 100 and the radiation source 1601 may be configured to scan thehuman in a linear or rotational direction.

FIG. 13 schematically shows a radiation computed tomography (RadiationCT) system. The radiation CT system uses computer-processed radiationsto produce tomographic images (virtual “slices”) of specific areas of ascanned object. The tomographic images may be used for diagnostic andtherapeutic purposes in various medical disciplines, or for flawdetection, failure analysis, metrology, assembly analysis and reverseengineering. The radiation CT system comprises the radiation detector100 described herein and a radiation source 1701. The radiation detector100 and the radiation source 1701 may be configured to rotatesynchronously along one or more circular or spiral paths.

The radiation detector 100 described here may have other applicationssuch as in a radiation telescope, radiation mammography, industrialradiation defect detection, radiation microscopy or microradiography,radiation casting inspection, radiation non-destructive testing,radiation weld inspection, radiation digital subtraction angiography,etc.

The electronics layer 120 in the radiation detector 100 may include anelectronic system 121 suitable for processing or interpreting orcorrecting signals generated by particles of radiation incident on thepixels 150 comprising radiation absorption layer 110. The electronicsystem 121 may include an analog circuitry such as a filter network,amplifiers, integrators, and comparators, or a digital circuitry such asa microprocessor, and a memory. The electronic system 121 may includecomponents shared by the pixels or components dedicated to a singlepixel. For example, the electronic system 121 may include an amplifierdedicated to each pixel and a microprocessor shared among all thepixels. The electronic system 121 may be electrically connected to thepixels by vias 131. Space among the vias may be filled with a fillermaterial 130, which may increase the mechanical stability of theconnection of the electronics layer 120 to the radiation absorptionlayer 110. Other bonding techniques are possible to connect theelectronic system 121 to the pixels without using vias.

FIG. 14A and FIG. 14B each show a component diagram of the electronicsystem 121, according to an embodiment. The electronic system 121 mayinclude a first voltage comparator 301, a second voltage comparator 302,a counter 320, a switch 305, an optional voltmeter 306, a controller 310and a memory 330.

The first voltage comparator 301 is configured to compare the voltage ofat least one of the electric contacts 119B to a first threshold. Thefirst voltage comparator 301 may be configured to monitor the voltagedirectly, or calculate the voltage by integrating an electric currentflowing through the electrical contact 119B over a period of time. Thefirst voltage comparator 301 may be controllably activated ordeactivated by the controller 310. The first voltage comparator 301 maybe a continuous comparator. Namely, the first voltage comparator 301 maybe configured to be activated continuously and monitor the voltagecontinuously. The first voltage comparator 301 may be a clockedcomparator. The first threshold may be 5-10%, 10%-20%, 20-30%, 30-40% or40-50% of the maximum voltage one incident particle of radiation maygenerate on the electric contact 119B. The maximum voltage may depend onthe energy of the incident particle of radiation, the material of theradiation absorption layer 110, and other factors. For example, thefirst threshold may be 50 mV, 100 mV, 150 mV, or 200 mV.

The second voltage comparator 302 is configured to compare the voltageto a second threshold. The second voltage comparator 302 may beconfigured to monitor the voltage directly or calculate the voltage byintegrating an electric current flowing through the diode or theelectrical contact over a period of time. The second voltage comparator302 may be a continuous comparator. The second voltage comparator 302may be controllably activate or deactivated by the controller 310. Whenthe second voltage comparator 302 is deactivated, the power consumptionof the second voltage comparator 302 may be less than 1%, less than 5%,less than 10% or less than 20% of the power consumption when the secondvoltage comparator 302 is activated. The absolute value of the secondthreshold is greater than the absolute value of the first threshold. Asused herein, the term “absolute value” or “modulus” |x| of a real numberx is the non-negative value of x without regard to its sign. Namely, |x|=

$\left\{ \begin{matrix}{x,\mspace{6mu}\text{if}x \geq 0} \\{- x,\mspace{6mu}\text{if}\mspace{6mu} x \leq 0}\end{matrix} \right)\mspace{6mu}.$

The second threshold may be 200%-300% of the first threshold. The secondthreshold may be at least 50% of the maximum voltage one incidentparticle of radiation may generate on the electric contact 119B. Forexample, the second threshold may be 100 mV, 150 mV, 200 mV, 250 mV or300 mV. The second voltage comparator 302 and the first voltagecomparator 310 may be the same component. Namely, the system 121 mayhave one voltage comparator that can compare a voltage with twodifferent thresholds at different times.

The first voltage comparator 301 or the second voltage comparator 302may include one or more op-amps or any other suitable circuitry. Thefirst voltage comparator 301 or the second voltage comparator 302 mayhave a high speed to allow the system 121 to operate under a high fluxof incident particles of radiation. However, having a high speed isoften at the cost of power consumption.

The counter 320 is configured to register at least a number of particlesof radiation incident on the pixel 150 encompassing the electric contact119B. The counter 320 may be a software component (e.g., a number storedin a computer memory) or a hardware component (e.g., a 4017 IC and a7490 IC).

The memory 330 is configured to store the sets of weights associatedwith pixels 150, generated signals and corrected signals of pixels 150.The memory 330 may also be used to store temporary values or resultsduring corrections of signals, and to store programs, procedures, orfunctions of signal correction. The memory may be made of a plurality ofnonvolatile memory devices, such as flash memory.

The controller 310 may be a hardware component such as a microcontrolleror a microprocessor. The controller 310 is configured to start a timedelay from a time at which the first voltage comparator 301 determinesthat the absolute value of the voltage equals or exceeds the absolutevalue of the first threshold (e.g., the absolute value of the voltageincreases from below the absolute value of the first threshold to avalue equal to or above the absolute value of the first threshold). Theabsolute value is used here because the voltage may be negative orpositive, depending on whether the voltage of the cathode or the anodeof the diode or which electrical contact is used. The controller 310 maybe configured to keep deactivated the second voltage comparator 302, thecounter 320 and any other circuits the operation of the first voltagecomparator 301 does not require, before the time at which the firstvoltage comparator 301 determines that the absolute value of the voltageequals or exceeds the absolute value of the first threshold. The timedelay may expire before or after the voltage becomes stable, i.e., therate of change of the voltage is substantially zero. The phase “the rateof change of the voltage is substantially zero” means that temporalchange of the voltage is less than 0.1%/ns. The phase “the rate ofchange of the voltage is substantially non-zero” means that temporalchange of the voltage is at least 0.1%/ns.

The controller 310 may be configured to activate the second voltagecomparator during (including the beginning and the expiration) the timedelay. In an embodiment, the controller 310 is configured to activatethe second voltage comparator at the beginning of the time delay. Theterm “activate” means causing the component to enter an operationalstate (e.g., by sending a signal such as a voltage pulse or a logiclevel, by providing power, etc.). The term “deactivate” means causingthe component to enter a non-operational state (e.g., by sending asignal such as a voltage pulse or a logic level, by cut off power,etc.). The operational state may have higher power consumption (e.g., 10times higher, 100 times higher, 1000 times higher) than thenon-operational state. The controller 310 itself may be deactivateduntil the output of the first voltage comparator 301 activates thecontroller 310 when the absolute value of the voltage equals or exceedsthe absolute value of the first threshold.

The controller 310 may be configured to cause at least one of the numberregistered by the counter 320 to increase by one, if, during the timedelay, the second voltage comparator 302 determines that the absolutevalue of the voltage equals or exceeds the absolute value of the secondthreshold.

The controller 310 may be configured to cause the optional voltmeter 306to measure the voltage upon expiration of the time delay. The controller310 may be configured to connect the electric contact 119B to anelectrical ground, so as to reset the voltage and discharge any chargecarriers accumulated on the electric contact 119B. In an embodiment, theelectric contact 119B is connected to an electrical ground after theexpiration of the time delay. In an embodiment, the electric contact119B is connected to an electrical ground for a finite reset timeperiod. The controller 310 may connect the electric contact 119B to theelectrical ground by controlling the switch 305. The switch may be atransistor such as a field-effect transistor (FET).

The controller 310 may be configured to perform the signal correction byreading the sets of weights from the memory 330, executing programs orprocedures stored in the memory 330.

In an embodiment, the system 121 has no analog filter network (e.g., aRC network). In an embodiment, the system 121 has no analog circuitry.

The voltmeter 306 may feed the voltage it measures to the controller 310as an analog or digital signal.

The electronic system 121 may include an integrator 309 electricallyconnected to the electric contact 119B, wherein the integrator isconfigured to collect charge carriers from the electric contact 119B.The integrator 309 can include a capacitor in the feedback path of anamplifier. The amplifier configured as such is called a capacitivetransimpedance amplifier (CTIA). CTIA has high dynamic range by keepingthe amplifier from saturating and improves the signal-to-noise ratio bylimiting the bandwidth in the signal path. Charge carriers from theelectric contact 119B accumulate on the capacitor over a period of time(“integration period”). After the integration period has expired, thecapacitor voltage is sampled and then reset by a reset switch. Theintegrator 309 can include a capacitor directly connected to theelectric contact 119B.

FIG. 15 schematically shows a temporal change of the electric currentflowing through the electric contact 119B (upper curve) caused by chargecarriers generated by a particle of radiation incident on the pixel 150encompassing the electric contact 119B, and a corresponding temporalchange of the voltage of the electric contact 119B (lower curve). Thevoltage may be an integral of the electric current with respect to time.At time t₀, the particle of radiation hits pixel 150, charge carriersstart being generated in the pixel 150, electric current starts to flowthrough the electric contact 119B, and the absolute value of the voltageof the electric contact 119B starts to increase. At time t₁, the firstvoltage comparator 301 determines that the absolute value of the voltageequals or exceeds the absolute value of the first threshold V1, and thecontroller 310 starts the time delay TD1 and the controller 310 maydeactivate the first voltage comparator 301 at the beginning of TD1. Ifthe controller 310 is deactivated before t₁, the controller 310 isactivated at t₁. During TD1, the controller 310 activates the secondvoltage comparator 302. The term “during” a time delay as used heremeans the beginning and the expiration (i.e., the end) and any time inbetween. For example, the controller 310 may activate the second voltagecomparator 302 at the expiration of TD1. If during TD1, the secondvoltage comparator 302 determines that the absolute value of the voltageequals or exceeds the absolute value of the second threshold V2 at timet₂, the controller 310 waits for stabilization of the voltage tostabilize. The voltage stabilizes at time t_(e), when all chargecarriers generated by the particle of radiation drift out of theradiation absorption layer 110. At time t_(s), the time delay TD1expires. At or after time t_(e), the controller 310 causes the voltmeter306 to digitize the voltage and determines which bin the energy of theparticle of radiation falls in. The controller 310 then causes thenumber registered by the counter 320 corresponding to the bin toincrease by one. In the example of FIG. 15 , time t_(s) is after timet_(e); namely TD1 expires after all charge carriers generated by theparticle of radiation drift out of the radiation absorption layer 110.If time t_(e) cannot be easily measured, TD1 can be empirically chosento allow sufficient time to collect essentially all charge carriersgenerated by a particle of radiation but not too long to risk haveanother incident particle of radiation. Namely, TD1 can be empiricallychosen so that time t_(s) is empirically after time t_(e). Time t_(s) isnot necessarily after time t_(e) because the controller 310 maydisregard TD1 once V2 is reached and wait for time t_(e). The rate ofchange of the difference between the voltage and the contribution to thevoltage by the dark current is thus substantially zero at t_(e). Thecontroller 310 may be configured to deactivate the second voltagecomparator 302 at expiration of TD1 or at t₂, or any time in between.

The voltage at time t_(e) is proportional to the amount of chargecarriers generated by the particle of radiation, which relates to theenergy of the particle of radiation. The controller 310 may beconfigured to determine the energy of the particle of radiation, usingthe voltmeter 306.

After TD1 expires or digitization by the voltmeter 306, whichever later,the controller 310 connects the electric contact 119B to an electricground for a reset period RST to allow charge carriers accumulated onthe electric contact 119B to flow to the ground and reset the voltage.After RST, the system 121 is ready to detect another incident particleof radiation. If the first voltage comparator 301 has been deactivated,the controller 310 can activate it at any time before RST expires. Ifthe controller 310 has been deactivated, it may be activated before RSTexpires.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

What is claimed is:
 1. A method comprising: obtaining a signal at apixel in an array of pixels of a radiation detector, wherein the signalis generated from radiation incident on the radiation detector;obtaining a corrected signal by correcting the signal with a combinationof a set of reference signals generated from the radiation at a set ofreference pixels in the array, wherein a set of weights are respectivelyapplied to the set of reference signals in the combination; and formingan image based on the corrected signal; wherein the set of weights is afunction of a location of the pixel with respect to the array.
 2. Themethod of claim 1, wherein each pixel in the array of pixels encompassesa portion of a radiation absorption layer of the radiation detector. 3.The method of claim 2, wherein the signal is generated from chargecarriers produced in the radiation absorption layer by the radiation. 4.The method of claim 1, wherein the set of weights is a function ofrelative positions of the set of reference pixels with respect to thepixel.
 5. The method of claim 1, wherein the pixel is a member of theset of reference pixels.
 6. The method of claim 1, wherein the radiationis X-ray or gamma ray.
 7. The method of claim 1, wherein the signal andthe set of reference signals are generated during the same time period.8. The method of claim 1, wherein the combination is a sum of the set ofreference signals with the set of weights applied thereto.
 9. The methodof claim 1, wherein the signal represents an intensity of the radiationat the pixel.
 10. A computer program product comprising a non-transitorycomputer readable medium having instructions recorded thereon, theinstructions when executed by a computer implementing a method ofclaim
 1. 11. A system comprising: a radiation detector configured togenerate a signal at a pixel in an array of pixels of the radiationdetector from radiation incident on the radiation detector; a processorconfigured to obtain a corrected signal by correcting the signal with acombination of a set of reference signals generated from the radiationat a set of reference pixels in the array, wherein a set of weights arerespectively applied to the set of reference signals in the combination;wherein the processor is configured to form an image based on thecorrected signal; wherein the set of weights is a function of a locationof the pixel with respect to the array.
 12. The system of claim 11,wherein each pixel in the array of pixels encompasses a portion of aradiation absorption layer of the radiation detector.
 13. The system ofclaim 12, wherein the signal is generated from charge carriers producedin the radiation absorption layer by the radiation.
 14. The system ofclaim 11, wherein the set of weights is a function of relative positionsof the set of reference pixels with respect to the pixel.
 15. The systemof claim 11, wherein the pixel is a member of the set of referencepixels.
 16. The system of claim 11, wherein the radiation is X-ray orgamma ray.
 17. The system of claim 11, wherein the signal and the set ofreference signals are generated during the same time period.
 18. Thesystem of claim 11, wherein the combination is a sum of the set ofreference signals with the set of weights applied thereto.
 19. Thesystem of claim 11, wherein the signal represents an intensity of theradiation at the pixel.